1. Field of the Invention
The present invention relates to a composite having superconducting property and a process for manufacturing the same. Particularly, it relates to a process for manufacturing a superconducting composite, such as a wire having higher and stable critical temperature and critical current density.
2. Description of the Related Art
Under the superconducting condition, the perfect diamagnetism is observed and no difference in potential is observed for all that an electric current of a constant finite value is observed internally, and hence, a variety of applications of superconductivity have been proposed in a field of electric power transmission as a mean for delivering electric power without loss.
The superconductivity can be utilized in the field of power electric applications such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation such as magnetic levitation trains or magnetically propelling ships; a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like, in the medical field such as high-energy beam radiation unit, in the field of science such as NMR or high-energy physics; or in the field of fusion power generation.
In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a Josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power.
However, their actual usage have been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge which showed the highest Tc could not exceed 23.2 K. at most. This means that liquidized helium (boiling point of 4.2 K.) is only one cryogen which can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also require a large-scaled system for liquefaction. Therefore, there had been a strong demand for another superconducting materials having higher Tc. But no material which exceeded the abovementioned Tc had been found for all studies for the past ten years.
It has been known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These type superconductors, however, possess a rather lower transition temperature of about 10 K. and hence usage of liquidized helium (boiling point of 4.2 K.) as cryogen is indispensable to realize superconductivity.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986)189]
This new oxide type superconducting material is [La, Ba].sub.2 CuO.sub.4 or [La, Sr].sub.2 CuO.sub.4 which are called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30 K. which are extremely higher than the known superconducting materials and hence it becomes possible to use liquidized hydrogen (b.p.=20.4 K.) or liquidized neon (b.p.=27.3 K.) as a cryogen which bring them to exhibit the superconductivity.
It was also reported in the news parer that C. W. Chu et al discovered in the United States of America another type of superconducting material having the critical temperature of in the order of 90 K. in February 1987, and hence possibility of existence of high-temperature superconductors have burst on the scene.
However, the above mentioned new type superconducting materials which was just born have been studied and developed only in a form of sintered bodies as a bulk produced from powders but have not been tried to be shaped into a wire form. The reason is that the new type superconductors are ceramic materials of compound oxide which do not possess enough plasticity or can not be worked easily in comparison with well-known metal type superconducting materials such as Ni-Ti alloy, and therefore they can not or are difficult to be shaped or deformed into an elongated article such as a wire by conventional technique such as wire-drawing technique in which superconducting metal is drawn directly or in embedded condition in copper into a wire form.
Still more, the above mentioned sintered ceramic materials must be shaped into an elongated structure when they are used as a superconducting wire in practice. However, the above mentioned superconducting materials obtained in a from of a sintered body are very fragile and are apt to be broken or cracked under even very weak mechanical stress. And hence, when they are shaped into a wire, special attention must be paid for their handling in order not to be broken.
It is proposed in Japanese patent laid-open No. 61-131,307 a method for manufacturing a superconducting wire from a metal type superconducting material which is apt to be oxidized and very fragile such as PbMo.sub.0.35 S.sub.8, comprising charging the material metal powder in a metal shell, extruding the metal shell filled with the material powder at a temperature of higher than 1,000.degree. C., and then drawing the extruded composite. This metal working technique, however, can not apply directly to ceramic material consisting of compound oxide, because the compound oxide type superconducting materials can not exhibit the superconductivity if not the specified or predetermined crystal structure is realized. In other words, a superconducting wire which shows higher critical temperature and higher critical current density and which is useable in actual applications can not be obtained outside predetermined optimum conditions. In particular, if the shell is not selected from proper materials, the resulting compound oxide will be reduced due to chemical reaction with the metal of the shell, resulting in poor or inferior properties of superconductivity.
A polycrystal having completely uniform crystal structure can not be obtained from particles having superconducting properties alone. Still more, the phenomenon of superconductivity is apt to be easily broken in stronger magnetic field and under the fluctuation or unhomogeneous distribution of temperature in the sintered body as well as the abovementioned oxide type superconducting materials possess rather higher specific resistance and lower heat-conductivity above the critical temperature. Therefore, if the phenomenon of superconductivity breaks locally, the sintered body produces Joule heat caused by the superconducting current preserved therein and explosive evaporation of cryogen is induced when the heated portion of the sintered body contacts with the cryogen. In order to avert this danger, in conventional metal type superconducting material, superconducting metal is shaped in a form of a fine wire or filament a plurality of which are embedded in electroconductive metal which play a roll of a by-pass of electric current when superconductivity break.
The oxide type superconducting materials are, however, difficult to be shaped or deformed into such filaments, because they have not enough plasticity or processability in comparison with well-known metal type superconducting materials such as Ni-Ti alloy.
In order to realize a reliable and practical superconducting structure, it is indispensable that the structure possesses enough strength and tenacity which is sufficient to endure bending force during usage and also has as finer cross sectional dimension as possible in such manner that it can transmit currency at higher critical current density and at higher critical temperature. However, conventional techniques can not or are difficult to produce wire shaped ceramic articles possessing satisfactory mechanical strength and tenacity as well as a higher dimensional ratio of length to cross section.
Taking the abovementioned situation into consideration, the present inventors have proposed processes for producing sintered ceramic wires having a practically usable higher dimensional ratio of length to cross section without using organic binder which is a cause of deterioration of strength and tenacity in U.S. patent application Ser. No. 152,713 titled "Process for manufacturing a superconducting wire of compound oxide-type ceramic" filed in Feb. 5, 1988 and Ser. No. 161,480 titled "Process for manufacturing a compound oxide-type superconducting wire" filed in Feb. 28, 1988 in which a metal pipe filled with material powder is subjected to plastic deformation such as wire-drawing technique by means of a die and then is sintered.
These solutions are themselves satisfactory but the present inventors has continued to develope another process which can produce a ceramic wire possessing higher strength and no breakage and complete the present invention.
Therefore, an object of the present invention is to overcome the abovementioned problems of the conventional technique and to provide an improved process for producing a superconducting wire-like composite which has a higher Tc and higher stability as superconductor which can be applicable to practical uses.